In monitoring the physiological condition of a patient, it is often desirable to display the variations in systolic, mean and diastolic blood pressures that have occurred over a number of hours. Systolic pressures are produced when the heart contracts so as to produce maximum pressure peaks in the blood pressure signal, and diastolic pressures are produced when the heart relaxes so as to produce minimum pressure peaks in the blood pressure signal. Ideally, there would be one maximum peak and one minimum peak for each heart cycle, so that the respective averages of the amplitudes of the maximum and minimum peaks occurring during a time period, such as a minute, could be plotted to form what is known as a "trend plot". In practice, however, the blood pressure signal contains peaks due to noise that have no physiological significance, so that their inclusion in the averages is a source of error. The effect of noise peaks can be reduced by signal processing means such as set forth in my copending U.S. Pat. application Ser. No. 894,679 filed concurrently herewith and entitled Derivation of Steady Values of Blood Pressures, and reasonably steady values of the peak blood pressures produced, but the values can be affected by continued bursts of noise. Furthermore, certain medical procedures cause interruptions in the blood pressure signal that make the average pressures derived from it unreliable.